Harmonic control system



April 2, 41946. A. BAILEY HARMONIC CONTROL SYSTEM Filed July 1; 194:

Receiver 1 1? Fa Fm Limiter Wi /Q III- - Receiver limiter TranslatorDetector IZ'JTn I.

I IN VI ZNTOR i 1 9 m A Patented Apr. 2, 1946 HARMONIC CONTROL SYSTEMAustin Bailey, Maplewood, N. J., asslgnor to American Telephone andTelegraph Company, a corporation of New York Application July- 1, 1943,Serial No. 493,007

12 Claims. (01. 178-44) v This invention relates to arrangements forautomatically controlling an element or elements of a system such as aradio or wire communication system.

In the art of communication, instances frequently arise in which it isdesirable to control a given transmission circuit or some othertransmission circuit in accordance with transmission conditions existingin a given circuit. For example, it has been common practice to usetransmission entering a path transmitting in one direction, to changethe condition of a path transmitting in the opposite direction bydisabling it, by rendering it capable of transmitting or by changing thevolume level of the path. It has also been common practice to use thetransmission entering a given transmission path to render such pathcapable of transmission where it is normally disabled, to disable itwhere it is in normal transmitting condition, or to change the volume oftransmission through the circuit. In other instances, some element in atransmission path is used in response to the volume level oftransmission in that path to control some other element in the path tokeep the transmission constant. As another example, in diversityreception, the signals in one or more radio receivers have been used todetermine which of the several available receivers should be used at agiven time,

Heretofore, in the various types of systems above outlined, the controlof some element in a produce some desired result, such as disabling thecircuit, rendering it capable of transmission,

changing the volume level, or selecting a desired circuit from agroup ofavailable circuits.

The invention will now be more fully understood from the followingdescription thereof, when readin connection with the accompanyingdrawing, in which Figure 1 shows the invention applied to the selectionof a preferable receiver in a radio system employing diversityreception; Fig. 2 shows the invention applied to a diversity receptionsystem operating on a slightly different basis, and Fig. 3 shows theinvention employed to control the gain of a preceding stage in a fre-.quency modulation radio receiver to insure proper limiting of thesignal at all times.

In a diversity receiving system it is desirable to select the receivingcircuit which at any particular timehas the strongest signals and is,therefore, most likely to give the best signal-tonoise ratio. Such asystem is shown in Fig. 1,

, and the harmonics appearing in the output 1 a directly heated cathodesand two anodes coninvention, however, it is proposed to exercise thecontrol not by means of the signal or a carrier upon which the signal issuperimposed, but by means of the harmonic condition in the output of alimiting device through which the signal (0 carrier) istransmitted.

Various types of limiting devices are known in the art which operate inresponse to alternating currents of varying amplitudes to limit theamplitude of a given alternating current wave to a preassigned maximumvalue; In a limiting device of this type, when an alternating currentwave exceeds the preassigned limit, the tops of the waves are, ineffect, cut off, thereby producing harmonics of the fundamental signalor carrier wave. In accordance with the present invention the volume ofthe harmonics appearing in the output of the limiting device is used tocontrol an element in a transmission circuit to nected in a circuit insuch a way as to prevent a received current wave from exceeding apreassigned limit.

In the system of Fig. 1,- two alternative receiving circuits aredisclosed, either of which may be used to transmit the received signalsto an outceiver REC, limiter TL, and pentode amplifier The limiter VL,as already stated, has two indirectly heated cathodes and two anodes sothat two separate transmission paths pass through the limiter. The twoanodes are connected together and a source of direct current B isbridged from the junction point of the two anodes through a resistance Zacross the transmission path to a conductor which is connected throughterminal apparatus to the two cathodes. A limiter of this general typeis disclosed and described in U. S. patent to Schelsinger, No.2,309,258, issued January 26, 1943. I

In the operation of a limiter of this type the current in the outputcircuit resulting from the voltage induced by the oscillations appliedto the circuit, cannot exceed in either direction the direct currentsupplied by the battery B to one of the anodes. Therefore, all higheramplitudes applied to the circuit are reduced to the maximumpredetermined by the battery supply. I1; will be seen that thebatterysupply circuit is, in eflect, bridged across the junction of aninput circuit I and an output circuit 0, each of which has a one-wayvacuum tube path included therein, the vacuum tubes being conductive inopposite directions; With such a connection a wave applied to the inputcircuit I produces a variable wave in the output circuit whose maximumamplitude will not be greater than the voltage between the outputcathode and the output anode produced bythe battery B. The currentflowing from the input cathode to the output anode acts, in eifect, as ashunt upon the path from the output cathode to the output anode of thedevice VL.

For example, when the signal wave received by the receiver REC is insuch a direction as to increase the normal current fiow between theinput cathode and the input anode of the device VL,

the current in the output 0 is decreased by the normal current fiow fromcathode to anode in the input circuit, and when it attains th same valuec is because the applied input wave opposes the as said normal currentor exceeds that value, no

current fiows from cathode to anode in the input path I, and. themaximum wave appearing in the output circuit 0 cannot attain a greatervalue than that produced by the voltage 01' the battery B. Consequently,the alternating current waves in the output circuit are limited or havetheir tops cut off so that they do not exceed a preassigned value.

As already stated, this results in producing harmonics in the outputcircuit 0 whose energy is determined by the degree of limiting of thereceived signal waves. The energy of the harmonics may then be used tocontrol elements in the receiving circuits of a diversity receptionsystem in such a way as to determine which of the receivers at a givenmoment will be efi'ective to transmit to the outgoing line L. In orderto obtain this result the output circuit 0 of the limiter is connectedthrough a low-pass filter LPF and over a signal path SP to' a pentodeamplifier PA whose output transformer T3 is connected to the line L. Thecircuit just described selects and transmits only s gna s of the desiredsignal frequency. The output circuit-0 is connected to an alternativepath through a high-pass filter HPF which passes the harmonics producedby the limiter VL through'a transformer T1 to a rectifier R.

This rectifier may be of any known type, but, as shown, is a tube havingtwo transmission paths between two indirectly heated cathodes and twocorresponding anodes. The paths through the stantially as follows: Thetwo pentode amplifiers aaeaaso tube are connected in the usual push-pullcircuit arrangement so that one-half of an A. C. wave passes through oneof the paths while the half of the A. C. wave of opposite sign goesthrough the other path of the rectifier. Thus both halves of the A. C.waves are rectified. In. the output circuit of the rectifier R is anetwork N of known type, which serves to smooth out the rectifiedcurrent so that in the circuit CC a rectified direct cuit CC connectedto the cathodes of the rectifier R, while the conductor connected to theanodes of the rectifier will have a positive voltage applied thereto.

The amplifier PA in the signal path SP involves a pentode type tubehaving an indirectly heated cathode C, an anode .A, a controlling screenor grid SC1, a second controlling screen or screen grid SC2, and asuppressor grid SG. The conductors of the path SP are connected to thecathode C and the screen SCI, respectively. The anode A is energized byD. C. potential from a battery B2 through a resistance T1 and isconnected through a transformer T3 to the output circuit L. Thesuppressor grid SG is connected to the cathode C and serves to pick upelectrons driven out of the anode A as secondary emission, thuspreventing such electrons from affecting the operation of the tube. Thenormal gain of the pentode amplifier PA is determined by a so-calledCpotential supplied by the battery B: through a resistance r to thecontrolling grid or screen S01.

Another receiving circuit, similar to that already described, isconnected to the antenna A and, as previously stated, includes a radioreceiver REC, a vacuum tube limiter VL', filters LPF and HPF', a pentodeamplifier PA, and a rectifier R, all of these elements being similar tothe corresponding elements associated with the antenna A, and also beingsimilarly connected.

The output of the pentode amplifier PA is con-' nected through atransformer T: to the outgoing follows: The positive side of therectifier R is connected to a junction pointb between the C-battery B3and the resistance r. The negative side of the rectifier R is connectedto the upper terminal a of the resistance r in the C-battery controlcircuit of the pentode amplifier P.A' Similarly, the positive side ofthe rectifier R is connected to the lower terminal b of the resistance rin the C-battery suppl of the pentode amplifier PA. The negative side ofthe rectifier R is connected to the upper or a terminal of theresistance r in the C-battery supply lead for the pentode amplifier PA.

Further control is efiected by connecting the screen grid SC: ofamplifier PA to the point 0' in the battery supply path of the plate-oranode of the pentode amplifier PA. Similarly, the screen grid SC: of thepentode amplifier PA is connected to the terminal 0 in the batterysupply path of the pentode amplifier PA.

The arrangement above described operates sub- PA and PA in the tworeceivers are adjusted with proper control bias supplied from thebatteries Ba and r, respectively, to the control grids 8C1 and SCr, sothat both amplifiers will have substantially equal amplification withoutany added voltage from the rectifiers R or R, as the case may be. If thelimiter VL is limiting the signals received from the antenna A, then'avoltage will be generated in the output of the rectifier R. Due to thenegative connection from the output of the rectifier to the point a'ofthe biasing control path to the control grid SCi of the amplifier PA,thisrectified voltage, which is proportional to the harmonics appearingin the output of the limiter VL, will increase the negative potential onthe control grid SCi' oi the pentode amplifier PA, thus reducing thegain of said amplifier.

The reduction in the gain of amplifierfA results in decreasing the platecurrent in s id pentode amplifier. Consequently, the positive potentialin the plate circuit will be increased at the point c'. This increase inpositive voltage in the plate circuit of the pentode amplifier PA willreact back on the screen grid SCz of the pentode amplifier PA toincrease the positive potential on said screen, thereby increasing thegain of the pentode amplifier PA. Furthermore, this reduces the positivepotential at the point c of the output circuit of the pentode amplifierPA so that it in turn reacts back on the screen SCz' of the pentodeamplifier PA to decrease the positive potential applied to said screenSCz'. This further reduces the gain through the pentode amplifier PA.The result is that the transmission in the receiving circuit connectedto the antenna A is reduced to the point where it is relativelyinefiective. while the amplifier connected in the circuit to the antennaA has its gain increased; Thus the harmonics produced by the limiter VLefiectively increase the output of the signal path SP and reduce orrender ineifective the output from the path SP.

If, on the other hand, the signals received from the antenna A are largeenough so that the effect of the limiter VL is to produce harmonics inits output path 0, the rectified potential across the terminals of therectifier R will in a similar manner decrease the amplification of thepentode amplifier PA and increase the amplification of the pentodeamplifier PA, Therethat path will be good. 0! course, if the signalsreceived in both paths are so poor that no limiting takes place ineither the limiter VL or the I limiter VL, the output from both pathswill be effectively transmitted to the line L. Under these circumstancesthe signal received by the line L will, of course, bepoor, but this isbecause neither receiver is receiving a signal of sufficient volume tobe eilective. With a system involving only two receivers such acondition may occasionaliy arise, but fortunately will seldom occur.

A modified arrangement is shown in Fig. 2. In this figure all of theelements are substantially the same as in Fig. 1 and are connected inasimilar manner, except that instead of pentode amplifiers PA and PAbeing employed, triode amplifiers .TA and TA of the type in which theoathode is indirectly heated are employed. Also'the connections from theoutput of the rectifiers R and R are reversed with respect to theconnections in Fig. 1. For example, the positive side of the rectifier Ris connected to the upper terminal a of the resistance r'in theC-battery supply circuit from battery B3 to the grid G of the triodeamplifier TA. The negative side of the rectifier R is similarlyconnected to the termifore, the receiver connected to the antenna A'will be effective to transmit to the outgoing line L.

If it should happen that the signals received on both antennae A and Awere exactly equal, then at that time both receivers will contributeequally to the outgoing circuit L. However, since the balance in such acase is unstable, a slight preponderance of the one received signal overthe other will result in the larger ,of the two signals taking controluntil the gain of the pentode amplifier, through which the strongersignal normally passes, is increased to the point where it has theeffect of substantially disabling the other pentode amplifier.

In general, the receiver which takes control will only be strong enoughto produce harmonics in the output of its limiter when the receivedsignal is considerably greater than the noise nor maliy appearing in thepath. The noise is usually made up of tube noise'and resistance noisegenerated within the receiver itself and is, therefore, fairly constant.Consequently, when the signal in one path is strong enough to producelimiting and take control, the signal-to-noise ratio in nal b of saidresistance 1. So also, the positive connection from the rectifier R ismade to the upper terminal a of the resistance r in the bias ing circuitof the triode amplifier TA, while the negative side of the rectifier Ris connected to the terminal b. of said resistance r.

The operation ofthe arrangement of Fig. 2 issomewhat as 'follows:Normally both triode amplifiers TA and TA are adjusted to the cut- 011point by the bias voltages supplied from the v v batteries B: and B3through the resistances r and r, respectively. In other words, bothamplifiers are normally adjusted to non-amplifying condition. When thesignals connected to the antenna A are strong enough to be limited bythe limiter VL so that harmonics are produced in the output circuitthereof, the harmonics are rectified by the rectifier R to produce avoltage which is applied across the terminals oi. the resistance r insuch a direction as to render the triode amplifier TA effective toamplify the received signal. In other words, the rectified voltage inthe path associated with the antenna A is applied in such a way as toreduce the normal biasing voltage upon the grid G of the amplifier, thuspermitting the amplifier to amplify signals. The amplifier TA in theother path, however, will still remain in its normally disabledcondition.

Similar action takes place in the lower receiver connected to theantenna A when the signal received in said branch is of sufficientamplitude to produce a limiting effect by means of the limiter VL. Theresultant rectified harmonics act to causethe triode amplifier TA toamplify while the other triode amplifier TA remains in its normallydisabled condition.

If the signals are strong enough in both receiving branches, both willcontribute substantially equally in the combined output circuit L.However, if the signal in one receiver decreases below a thresholdvalue, the biasing current on the grid of the corresponding triodeamplifier will be increased to the point where that amplifier becomes.disabled. Thus the corresponding receiving path is cut off completely,leaving only the alternate receiving path to pass signals to the outputcircuit L. i

Fig. 3 shows how the harmonics from a limiting device may be applied tocontrol thegain of a preceding stage in afrequency modulation radioreceiver to insure proper limiting at all times.

As is well known,-in frequency modulation the carrier is modulated bychanging its frequency rather than its amplitude in response to thecontrolling signal. Consequently, the carrier is sent out from thetransmitting station at a constant amplitude but at a varying frequency.Due to variable conditions in the transmission path to the receiver theamplitude of the carrier, as it is received, may vary from time to time.v It is therefore customary to arrange alimiter in a frequencymodulation receiving circuit to keep the received amplitude fromexceeding a predetermined maximum value, and to eliminate amplitudemodulatwo important particulars: (1) the received frequency issteppeddown to an intermediate fre- I quency before being detected, and (2) theintermediate frequency is applied 'to a volume'limiter (VL in Fig. 3) toproduce harmonics which are rectified to control the gain of anamplifier. Such harmonics may be used to maintain the received carrierat a substantially contant level in its intermediate frequency stage,regardless of variations in the transmitting path from transmittingstation to receiving station.

Referring to the apparatus of Fig. 3 in more detail, the receivingantenna A has associated therewith a high frequency receivingarrangement HFR. This functions to receive the high frequency, amplifyit if desired, and then convert it by stepping it down to anintermediate frequency. At the intermediate frequency the receivedenergy is passed through the transformer T4 to the input IFA. Thisamplifier may be of the triode type having an indirectly heated cathodeC, an anode A, and a controlling grid G. The output or plate circuit ofthe amplifier IF'A is connected through a transformer T5 to the input Iof a volume limiter VL which may be similar to the volume limiterspreviously described in connection with Figs. 1 and 2. However, it willbe understood that any known type of volume limiter may be utilized inthe circuits of Figs. 1, 2 and 3.

The output circuit 0 of the volumelimiter is connected through alow-pass filter-LPF to the other elements of the frequency modulationreceiving circuit which will be-described later-.- The output '0 ofthelimiter VL is also connected fier LFA'are employed, the output of thelatter being connected to the line L. The translator TR is a device sodesigned that when a constant amplitude wave of variable frequency isapplied thereto its response will vary with frequency in such a way asto produce an output wave changing in amplitude in accordance with theshifting frequency of the applied wave. Thus the frequency modulatedwave is in its intermediate frequency stage converted by the translatorTR into anintermediate frequency wave varying in amplitude in accordancewith the modulating signal applied to the distant transmitter. Thismodulated signal is then detected by the detector D, amplified by theamplifier LFA and applied to the outgoing line L.

The gain of the intermediate frequency amplifier IPA is controlled by anormal C voltage applied from the battery B: through the resistance 1'.When no limiting efiect results from the operation of the limiter VL noharmonics will appear in the output circuit 0 and the amplification ofthe amplifier-IFA will have the'maximum value determined by the biasingvoltage due to the batteryBs. However, if the amplitude of the signalreceived by the antenna A exceeds a limiting value, the received wave inits intermediate frequency stage is applied by the amplifier IFA to thelimiter VL which cuts down the amplitude of the received wave to apredetermined maximum, This limiter also produces harmonics in theoutput circuit 0 which are passed through the-filter HPF to therectifier R. The harmonic waves are now rectified by the rectifier R andapplied across thetemiinals of the resistance r through the circuit CCin such a manner as to increase the negative bias normally applied tothe grid G. This reduces the amplification of the intermediate frequencyamplifier and thus tends to keepthe signal applied to the translator TRcircuit of the intermediate frequency amplifier Within a predeterminedamplitude.

Since the control thus exercised is derived from the circuit at a pointfollowing the limiter VL, the gain control voltage of the intermediatefrequency amplifier IE'A will depend on the strength of the signal beinglimited. Consequently, the

receiver gain should always be adjusted to a point high enough toproperly operate the limiter at constant signal amplitude. If the signalexceeds this amount its effect will be to generate more harmonics andtherefore reduce the gain of the intermediate frequency amplifier IFA.If, however, the received signal isinsuflicient to produce through ahighpass filter HPF to a receiver R similar to the rectifier R of Figs.1 and 2 already described. The positive and negative terminals IFA willtend to increase to the, point where the gain reduction caused bytheharmonics in the output circuit will become effective.

' It will, of course, be obvious that the principle of utilizing theharmonics appearing in the output circuit of the limiter to control atransmission element in a transmission circuit may be applied to variousother types of circuits where gain control or other transmission controleffects are de sired.

It will also be obvious that while this invention has been disclosed incertain specific arrangements which are deemed desirable, the generalprinciples herein set forth may be embodied in many other organizations,widely different from those illustrated, without departing fromthespirit of the invention as defined in the appended claims.

What is claimed is:

1. In a signaling system, a transmission element associated with saidsystem, a volume limiter in said system, and means controlled solely byharmonics produced by the limiting action of said volume limiter tocontrol the eilectiveness of said transmission element.

2. In a signaling system, a signaling path, a transmission elementconnected in said transmission path, a volume limiter in said path, andmeans controlled by harmonics produced by the limiting action of saidvolume limiter to control the effectiveness oi said transmissionelement.

3. In a signaling system, a plurality of transmission paths, atransmission element in one of said paths, a volume limiter in anotherof said paths, and means controlled by harmonics produced by thelimiting action of said volume limiter to control the eflectiveness ofsaid transmission element.

4. In a signaling system, an amplifier asso ciated with said system, avolume limiter in said system, and means controlled solely by harmonicsproduced by the limiting action of said volume limiter to control thegain of said amplifier.

5. In a signaling system, a signaling path, an amplifier connected insaid transmission path, a volume limiter in said path, and meanscontrolled by harmonics produced by the limiting action of said volumelimiter to control the gain of said amplifier.

6. In a signaling system, a plurality of transmission paths, an'amplifier in one 0! said paths. a volume limiter in another of saidpaths, and means controlled solely by harmonics produced by the limitingaction of said volume limiter to control the gain of said amplifier.

7. In 'a signaling system, a transmission element associated with saidsystem, a volume limiter in said system, said volume limiter producingharmonics when an applied signal wave exceeds a predetermined amplitude,means to select and rectify the harmonics so produced, and meansresponding to the rectified current to control said transmissionelement.

8. In a signaling system, a signaling path, a transmission elementconnected in said transmission path, a volume limiter in said path, saidvolume limiter producing harmonics when an applied signal wave exceeds apredetermined amplitude, means to select and rectify the harmonics soproduced, and means responding to the rectified current to control saidtransmission element.

9. In a signaling system, a plurality of transmission paths, atransmission element in one of said paths, a volume limiter in anotherof said paths, said volume limiter producing harmonics when an appliedsignal wave exceeds a predetermined amplitude, means to select andrectify the harmonics so produced, and means responding solely to therectified harmonics to control said transmission element.

10. In a signaling system, an amplifier associated with said system. avolume limiter in said system, said volume limiter producing harmonicswhen an applied signal wave exceeds a predetermined amplitude, means toselect and rectify the harmonics so produced, and means responding tothe rectified current to control the gain of said amplifier.

11. In a signaling system, a signaling path, an amplifier connected insaidtransmission path, a volume limiter in said path, said volumelimiter producing harmonics when an applied signal wave exceeds apredetermined amplitude, means to select and rectify the harmonics soproduced, and means responding to the rectified current to control thegain of said amplifier.

12. In a signaling system, a plurality of transmissionpaths, anamplifier in one of said paths, a volume limiter in another of saidpaths, said volume limiter producing harmonics when an applied signalwave exceeds a predetermined amplitude, means to select and rectify theharmonics so produced, andmeans responding solely to the rectifiedharmonics to control the gain of said amplifier.

Y AUSTIN, BAILEY.

